A.L. Malinovsky

Intramolecular and intermolecular redistribution of vibrational energy in MP IR excitation: CF2HCl molecule

Abstract The dynamics of inter- and intramolecular vibrational energy redistribution for CF 2 HCl molecules, multiple-photon excited by means of a CO 2 laser, is studied by a Raman probing technique. Absolute values of absorbed energy for various molecular modes are measured. The intermolecular distribution exhibits two molecular ensembles — “hot” and “cold”, the former being characterized by a statistical energy distribution among all vibrational modes, including the high-frequency CH stretching one (ν 1 ). The reasons for this statistical intramolecular distribution and the possibility of mode-selective excitation using the MPE process are discussed. The dynamics of collisional establishment of the intermolecular equilibrium distribution function is studied.

Laser time-resolved raman spectroscopy of mode selectivity and vibrational energy distribution for IR MP excited polyatomic molecules

Abstract Intra- and intermolecular vibrational energy distribution in IR multiple-photon (MP) excited molecules is studied by a spontaneous Raman probing technique. The mode selectivity of IR MPE is shown to be limited by stochastization of vibrational motion. The existence of an internal energy threshold for the stochastization process is found. Values of this threshold and mode selectivity of IR MPE are measured for some molecules.

Vibrational relaxation of CH stretching modes in methane and its halogenated derivatives

Abstract Time-resolved Raman probing techniques have been used to investigate the collisional relaxation of CH vibrations in CH 4 CHF 3 and CHF 2 Cl. It has been shown for CH 4 that there is exchange initially between the ν 1 and ν 3 modes, and then a transfer to deformation vibrations occurs. The related constants have been measured. The relaxation in CHF 3 and CHF 2 Cl is found to be abnormally fast, the time constants p τ are 60 +20 −10 ns Torr and 30 +10 −5 ns Torr, respectively, which is less than the time of a gas-kinetic collision. Possible causes of such rapid relaxation are discussed.

Intramolecular vibrational dynamics of propyne and its derivatives: The role of vibrational-rotational mixing

The dynamics of intramolecular vibrational energy redistribution from the initially excited νHC mode in the propyne molecule (H-C≡C-CH3), as well as in three its derivatives that are obtained by replacing one of the hydrogen atoms of the methyl group with the chlorine atom (propargylchloride), the OH radical (propargyl alcohol), or with the NH2 radical (propargylamine), has been studied. Probing was performed by anti-Stokes spontaneous Raman scattering. The measured values of the deexcitation rate W of the νHC mode lie in the range 109−1010 s−1. A significant feature of the dynamics—an incomplete energy redistribution from the νHC mode—is especially clearly pronounced for the H-C≡C-CH3 and H-C≡C-CH2Cl molecules, for which the values of the relative level σ of the residual energy in the νHC mode are approximately equal to 0.54 and 0.25, respectively. A theoretical analysis performed made it possible to relate the parameters W and σ, on the one hand, and the density ρ of the so-called bath states, which are responsible for the vibrational energy redistribution, on the other hand. It is shown that, for all the four molecules considered, the required values of ρ can be accounted for solely by a strong vibrational-rotational mixing in the bath, as a result of which the projection of the total angular momentum onto the axis of the molecule ceases to be “good” quantum number.

A novel feature of intramolecular vibrational redistribution in propargyl alcohol and propargyl amine

Intramolecular vibrational redistribution (IVR) from the terminal acetylene mode nu(HC) has been studied for four molecules: H-C[Triple Bond]C-CH(3) (propyne), H-C[Triple Bond]C-CH(2)Cl (propargyl chloride), H-C[Triple Bond]C-CH(2)OH (propargyl alcohol), and H-C[Triple Bond]C-CH(2)NH(2) (propargyl amine). The experiments were performed with the room-temperature gases. The transition mid R:0-->mid R:1 in the mode nu(HC) was pumped by a short laser pulse. Anti-Stokes spontaneous Raman scattering was used as a probe. The measured parameters were the de-excitation rate W and the dilution factor sigma defined as the relative level of the residual energy in the nu(HC) mode at long pump-probe delay times. The pair of these values {W,sigma} allowed us to determine the density rho(eff) of those vibrational-rotational states, which are involved in IVR from state mid R:1. For two molecules, HCCCH(3) and HCCCH(2)Cl, the experimental results were consistent with the suggestion that all close vibrational-rotational states with the same total angular momentum J and symmetry participate in the IVR regardless of the other rotator quantum number K (in the case of HCCCH(3)) or K(a) (in the case of HCCCH(2)Cl) and the vibrational quantum numbers as well. For the other two molecules, HCCCH(2)OH and HCCCH(2)NH(2), this effect was also present, yet the experimental results revealed certain restrictions. We have obtained a satisfactory theoretical fit with the assumption that the low-frequency torsion vibration of the hydrogen atom in the hydroxyl group (in the case of HCCCH(2)OH) or hydrogen atoms in the amine group (in the case of HCCCH(2)NH(2)) does not participate in the IVR. This assumption can be treated as a challenge to future studies of these molecules by high-resolution spectroscopy and various double-resonance and pump-probe techniques.

Fast collision-induced redistribution of vibrational energy in halogenated methanes

Abstract Abnormally fast relaxation of the high-frequency CH stretch mode of CHF 2 Cl and CHF 3 molecules in mixtures with different buffer gases is observed. The probability of this process with respect to hard-sphere collision rate reaches 0.38 for the noble buffer gas (CHF 2 Cl:Kr mixture) and 1.03 for the polar buffer gas (CHF 2 Cl:SO 2 mixture). This result is referred to the collision-induced intramolecular vibrational relaxation (CIIVR) leading to the redistribution of vibrational energy from the initially excited mode among the nearby optically ‘dark’ states. The mechanisms resulting in CIIVR are discussed. It is shown that the high efficiency of this process results from the joint action of intra- and intermolecular interactions. The number of optically ‘dark’ vibrational states involved in CIIVR is experimentally measured for the CHF 2 Cl molecule. The obtained value coincides with the number of three-frequency combination states in the vicinity of the initially excited state.

Raman probing of overtone and combination bands to study the vibrational energy distribution produced by multiple-photon excitation

Abstract Vibrational energy distribution in CF 3 Br, produced by multiple-photon excitation, is studied with the use of Raman probing of fundamental bands and also overtone and combinations bands. On a collision-free time scale, statistical energy distribution among vibrational modes is found at energies over 7200 cm −1 . Possible physical causes of this effect are discussed.

Slow intramolecular vibrational redistribution: the latest results for trifluoropropyne, a comparison with the other terminal acetylenes and the mechanism*

We studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode ν1≈3330 cm−1 (acetylene-type H–C bond) in molecules in the gaseous phase by means of time-resolved anti-Stokes spontaneous Raman scattering. The time constant of this process was estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. We have compared this result with earlier results on the other terminal acetylene molecules, and give an explanation of this low IVR rate. Our suggestion for it follows from an assumption that the most probable doorway state leading to IVR from to the bath of all vibrational–rotational states consists of one quantum of the stretch and two quanta of the bend, and the matter is that the energy defect of Fermi resonance is essentially larger in trifluoropropyne than in other similar molecules. In addition, we have obtained the rate of collision-induced IVR for trifluoropropyne from experiments with various gas pressures and have shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational–rotational mixing.

Inter- and Intramolecular Vibrational Distribution in IR Multiple Photon Excitation: CF2Cl2 Molecule

Vibrational energy distribution of IR MP-excited CF2Cl2 is studied when pumping molecules through ν1 and ν8 modes. In both cases the intermolecular distribution is found to be in a state of nonequilibrium consisting of ensembles of “hot” and “cold” molecules. The structure of the “cold” ensemble is different when ν1 and ν8 modes are pumped. Statistical intramolecular energy distribution caused by stochastization of vibrational motion is found for “hot” molecules. The estimated value of stochastization onset energy equals Eth ≤ 7800 cm−1.

Extremely slow intramolecular vibrational redistribution: Direct observation by time-resolved raman spectroscopy in trifluoropropyne

We have studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode v1 ≈ 3330 cm−1 (acetylene-type H-C bond) in H-C≡C-CF3 molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The time constant of this process is estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. It is suggested that so long IVR time with respect to the other propyne derivatives can be explained by a larger defect, in this case, of the Fermi resonance of v1 with v2 + 2v7—the most probable doorway state leading to IVR from v1 to the bath of all vibrational-rotational states with the close energies. In addition, it is shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational-rotational mixing.